A widely accepted standard for controlling tolerances in mechanical components is set forth in the American National Standard for Dimensioning and Tolerancing, ANSI Y 14.5-1973, ASME, New York, 1973. One aspect of the standard deals with the location of feature patterns such as the pattern of holes formed in a mechanical component. In some instances, the location of a hole center relative to other holes within the same pattern is more critical than the location of the pattern as a whole. Thus, two independent sets of tolerances may be developed. One of these stipulates the tolerance on a hole center when measured relative to the other centers within a pattern. The resulting regions, wherein a hole center may be acceptably located, are defined as the feature relating tolerance zones (FRT zones). The regions generated by the other specified tolerances are defined as the pattern locating tolerance zones (PLT zones), and their function is to tolerance a hole center relative to a component's global datum. As defined in the above noted American National Standard, two acceptable schemes may be adopted for specifying the above noted zones, that is, composite positional tolerancing (CPT) and positional plus minus tolerancing (PPMT). A detailed description of these schemes is found in an article by J. R. O'Leary entitled "A Computer Simulation of a True Position Feature Pattern Gauge", 1981 Conference of Design Engineering Division of The American Society of Mechanical Engineers, 81-DE-9.
As pointed out in the O'Leary article, the need to control cost has propelled consideration of computer implemented systems for determining if mechanical components are within tolerances as a substitute for the use of mechanical gauges (often referred to as a go/no go gauge) or for the use of graphic pattern inspection of templates constructed by optical scanning of the mechanical component. However, such attempts at computer implemented systems have not been entirely successful. For example, the O'Leary article suggests a mathematical approximation to the use of a go/no go gauge which is based on the simulated movement of a pattern of measured features (such as hole centers on a mechanical component) relative to a dimensional pattern (desired pattern of hole centers) in order to minimize a response function based on a measure of the total of all vectors connecting corresponding measured and dimensioned features. After the simulated movement is effected, each of the measured features can be tested to see if the feature related tolerance has been satisfied.
While the proposed O'Leary response function leads to the acceptance of only mechanical components meeting the required tolerances, some good components might be missed since the response function does not necessarily result in optimally positioning the hypothetical "rigid body" but only approximates such optimal positioning. The O'Leary article does suggest that a more rigorous solution could be achieved by an iterative process wherein a measure of the total vectors is minimized in a constrained least squares sense which is said to be analogous to attaching linear restoring springs between the measured and dimensioned features and then seeking the resulting constrained equilibrium position. As pointed out in the O'Leary article, however, such a solution would require two levels of optimization and extensive computer calculations. O'Leary's remedy is to abandon the linear springs analogy in favor of an unconstrained system based on the use of highly non-linear restoring springs for purposes of deriving his response function. Moreover, to implement the O'Leary response function in a case in which positioning plus minus tolerancing is used, a second set of non-linear restoring springs with non-circular potential lines is required. Clearly, the O'Leary algorithm is only an approximate simulation of a mechanical gauge for testing tolerances for a pattern of features on mechanical components. Because the tolerance zones are considered only after the optimization search is finished, it is possible for a good part to be rejected as being out of tolerance. Also no provision is made for unusually shaped tolerance zone boundaries.
In U.S. Pat. No. 4,296,474 to Hurt an inspection system for inspecting plural features on machined parts is provided by a data processor programmed to cause a mathematical translation and rotation of a pattern of measured features into their "closest fit" with a pattern of dimensional (nominal) features after which the processor compares the measured features relative to the corresponding dimensioned features to determine if all measured features are within specified tolerances. The "best fit" of the Hurt patent is achieved by first computing the centroids of the pattern of measured and dimensioned features and, thereafter, causing a relative translation of the patterns to superimpose the centroids. Thereafter, the patterns are rotated relatively to minimize the sum of the areas defined by circles whose radii are equal to the distances between corresponding measured and dimensioned features. Obviously, a system of this type can not take into account the size and shape of the feature tolerance zones in determining a "best fit" and thus might reject a mechanical component which is, in fact, acceptable. Also this system does not allow for total movement of the pattern.
Numerous techniques are known for measuring the location of features on a mechanical component such as disclosed in U.S. Pat. Nos. 4,241,509; 4,221,053; 4,181,958 and 4,030,201. However, none of these patents suggest systems for simulating the repositioning of a pattern of measured features to a pattern of dimensioned features.
The prior art has, thus, failed to disclose a computer implemented inspection system for accurately simulating a go/no go gauge for determining if a pattern of features on a mechanical component are within tolerances and, if not, for determining if one or more of the features can be reworked to cause the resulting pattern of features to be within tolerance.